In the import frames section I think that it is just where the hotspot is automatically placed for each tile, this saves your time of setting the hotspot for each frame. I can't explain a hotspot well but if you have done transformations in mathmatics then you learn that all actions (like rotation or enlargement) need a center point to act around, a hotspot is just the "center point" of the object. So if you created the tiles with a center hotspot, then any co-ordinates you move the sprite to will have half of the image above the X, Y co-ordinate and the other half below.